Cosmological Revolution II: Copernicus
Copernicus' Motives for Developing his System
The Main Propositions of Copernicus' New System
Problems in Copernicus' System
Osiander's Preface and "Saving the Appearances"
It is with Nicholas Copernicus (1473-1543) that modern cosmology begins. Copernicus was born in Thorn, now part of Poland. In 1491 he went to study at the University of Cracow, which he left in 1494 without obtaining a degree. This university was known for its mathematics and astronomy, and it is believed that Copernicus studied those subjects, but no record appears. He returned to Thorn, and his family decided he should enter the Church, for which purpose he went in 1496 to study in Bologna. At the same time he was was appointed to fill a vacancy as canon of the cathedral chapel at Frauenberg, in Ermeland province, where his uncle was bishop since 1489. But he did not return to assume his post, and even when he did, 10 years later, his role in church affairs was minor and peripheral to his real interests-- astronomy and cosmology. At the university of Bologan he studied civil law; then he went on to Padua, he studied canon law and medecine. Curiously, he received his doctorate in canon law from yet a third unversity, Ferrara, in 1503. He returned to Poland in 1506, after spending some 10 years in Italian universities, studying maths, astronomy, classics, law and theology.
Copernicus first described his alternative system to that of Ptolemy in a privately circulated letter of 1514. A worked up version of this theory was published by his student George Rheticus in 1541, entitled First Account, followed by Copernicus' own exposition in De Revolutionibus Orbium Celestium (1543), published in the year of his death. Indeed, the story is well known that he received the printer's proofs on his death bed. He died never having seen the preface written by Osiander, which we will discuss momentarily. Copernicus also wrote a brief sketch of his system, which was not printed during his lifetime. After his death, its existence was unknown for some 3 centuries, and in 1878 a copy was found and published in Vienna. The work is usually entitled Commentariolus, or commentary. This sketch represents an earlier stage of his thinking, and though heliocentric, has some technical differences with the final, 1543 version.
Why did Copernicus develop his system? We have a brief account in his dedication to Pope Paul III. He notes that mathematicians differ among themselves on the subject of cosmology, with contending systems based on epicycles, eccentrics and equants. He takes this as an indication that something is amiss, for their is no agreed upon and sound set of basic principles. He next turned to the history of his subject, and found that at least some ancient authors had put the sun at the center and the earth moving around it. In I.5 he mentions a number of names, including that of a Pythagorean, Philolaus, who is generally credited with placing a central fire (not the sun, but certainly not the earth) at the center. But in the book as published he did not explicitly mention Aristarchus, who alone among the ancients had explicitly placed the sun at the center. Nonetheless, Copernicus felt himself entitled to experiment with a sun-centered (helioc-centric) universe.
"Occasioned by this [the views of ancient authors mentioned above] I also began to think of a motion of the earth, and although the idea seemed absurd, still, as others before me had been permitted to assume certain circles in order to explain the motions of the stars, I believed it would readily be permitted me to try whether on the assumption of some motion of the earth better explanations of the revolutions of the heavenly spheres might not be found. And thus, I have, assuming the motions which I in the following work attribute to the earth, after long and careful investigation, finally found that whent he motions of the other planets are referred to the circulation of the earth and are computed for the revoluton of each star, not only do the phenomena necessarily follow therefrom, but the order and magnitude of the stasrs and all their orbs and the heaven itself are so connected that in no part can anything be transposed without confusion to the rest and to the whole universe."
Besides the breach opened by debate over the Ptolemaic system, two other factors likely played a role in Copernicus' switch to a new system. (1) He was himself dissatisfied with the Ptolemaic system, in particular its deferents and equants, and wanted to remove them from astronomical theory. The ad-hoc nature of these mathematical "additions" was inelegant; the resulting system overly complicated. Perhaps matters could be cleared up by replacing the order of sun and earth.
(2) The Renaissance, particularly in Italy, witnessed a revival of Platonism. Copernicus had been exposed to this Platonic revival, which was associated in the arts with humanism and in mathematics with Pythagoreanism. Copernicus had most likely studied some Platonic philosophy in Italy during his studies there. In hisRepublic, Plato used two major metaphors Plato to represent his theory of how to achieve the supreme knowledge of the Form of the Good: the analogy of the line, and the analogy of the cave. What represents the Form of the Good in both is the Sun. Now in Plato's own system of cosmology, the sun appears as just one of the heavenly bodies revolving about the earth. But could it not be possible to more consistently Platonist than Plato! That would mean upgrading the status of the sun, to the center of the world, even if Plato himself had not done so.
Rheticus, who was the most important student of Copernicus, wrote a text called Narratio Prima (First Account) of Copernicus' system in 1540. This was the first published news of the system, a few years before Copernicus' own treatise. In it Reheticus discussed the problem of revising the physical views of Aristotle (the earth is no longer the center of the universe) and of Ptolemy (eccentrics and equants are no longer to be used) as follows:
"Now in physics as in astronomy, one proceeds as much as possible from effects and observations to principles. Hence I am convinced that Aristotle... if he could hear the reasons for the new hypotheses, would doubtless honestly acknowledge [them?].... I can therefore well believe that he would support my teacher, inasmuch as the well-known saying attibuted to Plato is certainly correct "Aristotle is the philospher of the truth". (Rosen, p. 142)
This is a typical formulation of the period: Aristotle would agree to the changes, even if (unstated here) present day Aristotileans fight it tooth and nail; Plato is invoked as an authority even over Aristotle himself, in a reference to a statement that does not occur in any of the existing dialogues, but which serves nicely the purposes Reticus pursues. Again, Rheticus says: "Following Plato and the Pythagoreans, the greatest mathematicians of that divine age, my teacher thought that in order to determine the causes of the phenomena circular motions must be ascribed to the spherical earth." (p. 148).
Copernicus in both his Commentariolus and his published treatise, On the Revolution of the Heavenly Orbs made explicit his basic assumptions, of which the following are the key assertions concerning the earth, sun, moon and stars:
1. The earth is not the center of orbit of all the heavenly bodies, but just of the moon; the earth moves and orbits the sun. (Thesis of "geo-dynamism" -- the earth moves --; as opposed to the ancient doctrine of "geo-statism" and "geo-centrism"- the earth as fixed at the center).
2. The sun is the center of orbit of all the heavenly bodies, with the exception of the moon. This completes Copernicus' most radical move: exchanging the place of the earth and sun. The sun is now held to be immoveable and the center around which the earth, planets and stars revolve (Thesis of "helio-centrism" and "helio-statism" -- the sun immoveable at the center --, as opposed to the ancient doctrine of "helio-dynamism" -- the sun moves around the center).
3. The stars are much futher away than previously supposed. This is not based on any observations, but is required by the new system of the earth orbiting the sun. In this new sy;stem, there should be a difference between the angle of observation of a star when observed six months apart -- at opposite points in the earth's orbit of the sun -- a phenomena known as "stellar parallax". However, this cannot be detected by the naked eye, and so Copernicus had to assume that the stars were so far away from the earth that the difference in angle for observations a half-year apart would be vanishingly small. More on this later.
4. The apparent daily motion of stars is due not their own motion around the earth, but the earth's daily rotation on its axis. The stars are conceived of as "fixed" in their outermost sphere. Similarly the "rising" and "setting of the sun" is only apparent; this appearance once more due to the daily rotation of the earth.
5. The apparent retrograde motion of the planets arises not from their really changing direction, but as a result of the relative differences in the position of the earth and the planets.
The development of Copernicus' theory does not follow an inductivist model of generalizing from novel observations. There were no new facts to support his system over that of Ptolemy's. Naked eye observations are indifferent to any choice between the two. Moreover, there was an observation that told against Copernicus and did not contradict Ptolemy: namely, the absence of stellar parallax. Hence, Copernicus' assumption that the stars are much further away than previously believed, again with no new observational data to back up his claim. (This is a kind of "saving the phenomena" assumption we had seen at work in ancient Greek cosmology). Indeed, Copernicus is believed to have made no more than 27 observations of his own, throughout his whole career! It was not factual considerations, to the contrary, but theoretical ones, that prompted his decision.
There is still another physical problem in the background to Copernicus; namely the problem of the lack of perceived motion of the earth. So long as Aristotilean physics was dominant, this problem too could not be adequately answered, except by making analogies to two boats in the water carried by the ripples: one does not seem to be moving relative to the other, but only relative to the shore. Analogies of this sort were in fact used by Copernicus, but they do not replace a true physical explanation.
Finally, Copernicus soon found that he had to use more and more epicyclic constructions to get his own system to square with observation, the problem being that he still used circular orbits, which introduce errors because of the fact that planetary orbits are really elliptic, a fact not known till some 3/4 of a century later with the work of Kepler.
We may wonder why Copernicus' system did not cause more fuss in conservative and church circles at the time than it did when once these ideas were contained in the De Revolutionibus. After all, the same enunciations will bring about Galileo's condemnation almost a century later. The main reason, is the preface to De Revolutionibus. Rheticus was supposed to see the book through printing, for Copernicus was by then old and infirm. But in 1542, Rheticus was appointed as a Prof. at Leipzig, and he passed on his task to Andreas Osiander, a Lutheran theologian, who completed the job. Osiander was most likely uneasy about the realistic stance of Copernicus' heliocentric, geodynamic theory, and added an anonymous preface, entitled "To the reader about the hypotheses of this work." For a long time, it was assumed that Copernicus had written the preface, just as he had written the dedication. Only Kepler some time later determined, through examination of letters written by Osiander at that time, that Osiander, not Copernicus was the author of the preface. This discovery itself was not published for quite some time afterwards, in 1858 (Stimson, p. 29).
What is said in the preface? "For it is not necessary that these hypotheses be true, nor even probable, but this alone is sufficent, if they show reasoning fitting the observations." Once again, the old saving the phemonena theory. Copernicus knew nothing of this. A copy of the printed book was given to him on May 24th, 1543, as he lay dying of paralysis. His life work was completed, but the controversy, which we will follow in the rest of the week, had just begun.
This preface deflected potentially damaging rejection by the church and conservative Aristotileans. It served as a sort of protective belt in Lakatosian terms. But the physical problems (no perceived stellar parallax, and especially no perceived terrestrial motion) were physical obstacles to the acceptance of the new theory, besides philosophical and theological ones which are evident. Opinion was divided, and for some time, throughout the 16th century in fact, the Copernicans were a definite minority.